Electromagnet



Feb. 17, 1942.

Filed Feb. 28, 1940 F. C. STOCKWELL 'ET AL ELECTROMAGNET 2 Sheets-Sheet1 f 4' c 5: INVENTORS ATTORNEYS Patented Feb. 17, 1942 ELECTROMAGNETFrank C, Stockwell, Hoboken, and William F. Penrose, Irvington, N. J.,assignors to Empire Electric Brake Corporation, a corporation ofDelaware Application February 28, 1940, Serial No. 321,168

Claims. 101. 188-164) This invention relates generally to an improvedcompact iron-clad flat-face central-core direct current electromagnet tobe operated in sliding friction engagement with a rotating disclikeflat-face armature plate and particularly to such an electromagnet'usefulin combination with a lever in electromagnetically actuatedbrakes wherein the actuatingmechanism including the electromagnet issubject to severe space limitations as, for example, the small freespace between brakejshoe assembly and axle within the drum ofconventional automobile, truck, trailer, and motorcycle brake drums.

The invention has for its principal object to so proportion thedimensions of an iron-clad flatface electromagnet structure or casing.including the'yoke,'the winding space, and the inner and outer cores,that within restrictions imposed ness at the center line of thefwindingspace to provide substantially uniform area of magnetic path orsubstantially uniform flux densityneglecting leakage, throughout themagnetic circuit.

We are aware that circular iron-clad centralcore direct currentliftingelectromagnets of high efficiency have been proposed and constructed,

for example, the so-called l-Iazeltine e1ectromag net described in thetext -Electrical Engineering, The Macmillan Company, 1924, by ProfessorL. A. Hazeltine. But prior to the present invention compact iron-cladcentral-core electro-- magnets were not known or indicated in the art.

Among other objects of the invention'are to provide a separate flatfacing for the electromagnet which se es to cover the winding space withmaterial having desirable friction characteristics and to providecoplanar flat magnetic material working pole faces over the inner andouter cores of the electromagnet casing contiguous the frictionmaterial. The facing i preferably so constructed of magnetic andnon-magnetic wear resistant friction material as to P mit of a highattractive force between electrotral and outer cores.

constant coeflicient of friction independent of relative motion betweenthe electromagnet and its armature and of the magnetic attraction normalforce between them. It is intended that the facing shall have areasonably long life in use although it is designed to be subject togreater wear than the armature.

An important object of the invention is to provide the electromagneticsystem, including the armature, with nicety of control of the magneticforce. This requires that the magnetic circuit shall, even at maximumcurrent in the winding, be saturated only to a limited extent. Thedesired condition may be obtained by interposing a non-magnetic gap orgaps of predetermined thickness within the system. To assure that a gapshall remain of fixed and uniform thickness,

' it may conveniently be established by the use of thin non-magneticsheet metal arranged within the electromagnet casing a .d facing at aplane not subject to wear, for example, at the back of the separatefacing upon either or both the cen- The presence of a nonmagnetic gapalso serves the important function of destroying residual magnetizationwith the result that the frictional force falls to zero when the directcurrent voltage sourceisdisconnected A from the electromagnet winding.

- with the moving armature, 'it has been proposed to apply theforcederived between the electromagnet and armature from within the centralcore of the electromagnet at a plane closely adjacent the fabe of theelectromagnet in sliding from within the central core closely adjacentthe' face thereof and substantially at axis of symmetry.

The invention may be morefully understood by reference to the drawingsand the following description. In the drawings:

or about the Fig. 1 is a front view in schematic central verticalsection showing the configuration of the electromagnet structure of thepresent invention with the inner and outer core pole faces in contactwith a flat-face armature plate fragment;

Fig. 2 is a front view in schematic central vermagnet and armature andto assure a substantially tical section of an electromagnet having anopeningin the centralcore in accordance with a feafrom Equation 6 andthus the dimensions n and ture of the present invention; 1': determined.The evaluation of c is preferably Fig. 3 is a. graph showing a series ofcurves remade from a graph where for each of several lating to Figs. 1and 2; values of above and below unity corresponding Fig. 4 is a frontview in vertical section at a 5 values of diameter of an electromagnetcasing having a A r; a separate facing and provided with a non-mag- '5or E netic gap of uniform fixed thickness, all in accordance with thepresent invention: are computed and plotted. For. example, let

Fig. 5 is a side view showing the electromagnet c=1'o then in operativesliding friction. engagement with a 18 and 3 239 rotating disc-likearmature plate and with the v a r: electromagnet winding connected withmeans ina Similarly f other values of The graph cluding a source ofvoltage for controllably enerf 0 versus gizing the electromagnet. l5 aFig. 6 is a front view in vertical section of a modification of Fig. 5,wherein the interposed non magnetic a of nommagnetic material is for acircular flat-face armature, direct current whouy disposed in themagnetic path within the electromagnet with solid central core as inFig.

centr core 2o 1 is shown as curve A in Fig. 3.

Referring now to Fig. 1, the variou dimensions One of the spacelimitations controlling the of the circular iron-clad central-coreelectromag- Proportlomng 0f the electromagnet in accordanw t casing l,are indicated by refere e letters with the invention is the availableover-all axial as follows: a equals yoke thickness at the centhicknesscasing Plus armature Plate thmk' ter line of the annular winding space2, and also 11855 the dimensiol} have the thickness .of the armatureplate The that the depth of windmg space h is preferably .depth of thewinding space 2, is indicated by 72.. related to the parameters a yokeor armature The radius of the central inner core is indicated platethickness a 0 dimensional index as by n, the inner radius of the outercore by n, lows: and the outer radius by T3. The yoke thicknesses at thechamfered portions 4, at the back 0 of the casing l, are indicated by mand a2, re- To accommodate th d ir d coil winding and sp c i yalso tokeep within specified temperature limi- The criterion adopted inaccordance with the mu 7;, if i d, b in r ased slightly inven i n s a eWinding Space is D D over the value determined by the above relationedwith -respect both to the outside dimentionship.

sums of the casing and to the yoke thickness at In Fig. 2 the variousdimensions of the electhe center line of the annular winding spacetromagnot as 8 corresponding to those of to provide substantiallyuniform area of magare indicated by the some reference netic path ters.Since the casing 8 is provided with an Thus axial circular opening 9, inthe central core ll, the radius thereof is indicated by 1'0. The V8131?!of To and T3 are specified, then similarly'to e gffgfifi fifig gz zgg if path. The development of the equations relating to Fig. l.

Let

From ,r(r1-ro)=1ra(n+rz) and the dimensional index Equation 2 above bedesignated the dimensional index (2) 1 cThen from I r o (1+ 1+?+al 3rrfl=m n+rzl and (2), f

From k 1 11 From 1+ck wri =1r(r;-ra") and (4), 2%(7 (5) From (7) and(2),

2 1an"=a(r1+ra) =c(1a -ri") From (3) (1(5) I v whence using (9) and(1111 k a 4 F" .-r=[ 1 ,12 2+%+ 2+? (6) 1 9 ck- J From (9) and (12) o Ifthe outer radius r: is specified, then, for 1+6, 2 K

any chosen value of armatureplate thickness i=kL( +1)( ck 13 a, thedimensional index 0 may be computed The dimensional index of Equation 13is preferably evaluated for specified values of rs and 1'0 and for aseries of values of a fronfa graph containing a family of curves of cversus The graph based upon Equation 13 is shown in Fig. 3 in curves Bto E inclusive.

The proportion of the dimensions of the electromagnet casing inaccordance with the invention, for example as schematically shown inFigs. 1 and 2, as determined by the dimensional index 0 is independentof the kind of ferromagnetic material used or of the desired magneticforce to be'exerted. To obtain a desired force when operated at aspecified voltage, suitable ferromagnetic material must be chosen, thehigher the desired force, the better must be the quality" orpermeability of the material or the more highly must the material besaturate In Fig. 4 the circular cylindrical ferromagnetic casing l5 ofthe type of Fig. 2 having a circular opening l6, axially the circularcentral core I1,

is provided with an annular winding l8, preferably of the pre-formed,insulated, and-iii regnated type. The winding l8 as preformed c qselyfits the annular winding space l9 and is firmly retained therein by athin annular washer 20 of nonmagnetic material, preferably. brass, inpress fit about the central core II on the annular shoulder 2| thereofand on the annular shoulder 22 of the outer core 23.

When the washer 20 is properly in place, the

' face of the casing l5 presents a substantially flat surface includingthe central and outer core faces 24 and 25 in coplanar relation. Overthe circular flat surface presented, by the central and outer core faces24 and 25 and the washer 20, there is arranged a thin circular disc orshim 26 of nonmagnetic material, preferably brass, for the purpose ofinterposing 'a fixed, non-magnetic gap or gaps of fixed uniformthickness sufficient to prevent substantial saturation of themagneticcir- .cuit even at maximum current in the winding thereof and to destroythe tendency of residual magnetization. As an alternative, a thin shim26a may be laid over but one of the core faces, for example, the centralcore as shown in Fig. 6,

in which event it will be of the order of twice the thickness tointerpose an equivalent non-magnetic gap to that provided by the shim 26as described.

Upon the shim 26 and in full fiat contact an inner circular portion 29,both of the same ferromagnetic material, preferably of high permeabilityand of softer material than that of 'the' armature plate to be used inconjunction with Theferromagnetic material usedfor the outer theelectromagnet. Contiguous the annular outer and circular inner portions20 and 29,there isprovided an annulus-of non-magnetic friction material30, preferably of the kind used in hard brake linings and having acoefllcient of friction of the order of 0.4.

It is to be noted that the members 28 and 23, are of smaller area thanthe face of the central and outer cores l1 and 23,for which the facing2'! provides coplananpole faces. In this way, the magnetic attractionbetween the electromagnet and armature is increased. when in operativefiat contact relation.

The facing 21 and shim 26 may be affixed to the casing l5 by machinescrews 3|, extending into the central core II, or in any other effectivemanner. When assembled with the casing ii, the separate facing 21 notonly permits of a high attractive force between electromagnet andarmature in fiat contact but also by reason of the non-magnetic frictionmaterial annulus 30, tends to assure a substantially constantcoeflicient of friction, independent of relative motion between theelectromagnet and armature and of the magnetic attraction normal forcebetween them. It is intended that the facing 21, shall be wear resistantand have a reasonably long life intuse although it isdesigned to besubject to greater wear than the armature employed with, theelectromagnet.

To provide electrical connection to the winding I8, two circularopenings 32 are provided in the yoke at the back of the magnetprovidingapertures leading to the bottom of the winding space 19. Toprovide against the possibility of moisture being admitted through theopenings 32, specially shaped soft rubber plug members 33 having acollar portion 34, and an elongated portion 35, depending therefrom, arefitted into the opening 32, at acountersunk portion 36, at the bottom ofthe winding space IS. The electrical leads from the winding l8, are ledthrough the rubber members 33, as will be observed.

The ampere turns to be employed in the winding l8 at a given operatingvoltage may be determined from equations well known in-the art, and thediameter of the wire employed in the winding will be determined largelyby the allowable temperature rise due to coil heating.

In electromagnets constructed in accordance with the invention, likeFig. 4, and adapted to be energized from 6-volt storage batteries,satisfactory casings have been made of low carbon hotrolled steels knownin the trade as SAE1112 and X1113. A compact electromagnet particularlyadapted for use in electromagnetically actuated motorcycle brakes hasbeen constructed in accordance with the invention with an outside cas--ing diameter of 1% inches and approximately inch in over-ail thicknessincluding the facing and a 0.002 inch brass shim. In this electromagnet,N0. 29 enameled wire is used in the winding and at 0.8 amperes thetangential force developed with its rotating armature is of the order of30 pounds. In a similar electromagnet for tlse in a'conventlonalautomobile ten-inch brake drum, the

electromagnet casing is. 2% inches in outside diameter and approximatelyVa inch in over-all thickness including the facing and a 0.003 inchbrass shim. In this magnet, No. 23 enameled wire is used in the windingand at 2.0 amperes,-

t e tangential force developed is of the order of pounds.

and inner magnetic material portions of the type of Fig. 4, is whollysupported on a working member (not shown) from within the opening 43, inthe central core closely adjacent the face thereof, and with its fiatface in continuous rubbing contact relation with the flat face of thearmature 40.

The winding of the electromagnet 42 is connected in circuit with asource of direct current voltage 44 and the rheostat 45. Uponmanipulation of the rheostat 45 the electromagnet winding may becontrollably energized from zero to any desired current value up to themaximum at I the operating voltage. For each current value there is acorresponding normal magnetic force and tangential frictional forcebetween the electromagnet and armature.

Where it may be desirable to fabricate the electromagnet casing byassembling parts thereof of twogrades of iron having different magneticproperties respectively, the area of magnetic path in the iron of lowerpermeability should be increased in the ratio or the maximum normalpermeabilities of the two grades of iron. Similarly, if the armatureplate material has a different permeability than the casing yokematerial, the thickness thereof should be different from that of theyoke thickness a in the ratio of the maximum normal permeabilities ofthe two materials. For example, if the armature plate material has alower permeability, its thickness should be greater than the yokethickness a in the ratio of the maximum normal permeabilities of the twomaterials. a

It is to be und'erstoodthat by use of the dimensional index 0 and thedevelopment of equations similar to those of (l) to (13), the in-.vention may be applied to electromagnets whose sections are notcircular cylindrical in shape.

We claim;

1. A compact iron-clad central-core flat-face direct currentelectromagnet adapted to be operplate, having a structure comprising, acircular cylindrical casing of ferromagnetic material providing a yokeand an annular winding space open at the face of the casing betweenconcentric central and outer cores, and having a circular openingthroughout the said central core concentric the axis of the'casing, thedepth and radial width of said winding space being proportioned' toprovide a substantially uniform area of magnetic path in said casing,said proportioning being determined by theradius of the said opening inthe central core and by both the outside diameter of said casing and theyoke thickness at the centerline of the winding space, a non-magneticgap of non-magnetic material interposed in the said magnetic path atsaid central core face, and a continuous circular separate facing ofmagnetic material arranged over said non-magnetic material insubstantial parallel relation with said central core face to provide aworking pole-face therefor, said interposed nonmagnetic gap be'ngadapted to assure nicety of electromagnet c ntrol and. to preventsubstantial residual magnetic attractionbetween elec-- direct currentelectromagnet adapted to be operated in continuous relative movementsliding friction engagement with a flat-face armature 'viding a yoke andan annular winding space open at theface of the casing betweenconcentric central and outer cores, and having a circular opening in thecentral core extending tltcrethrough concentric the axis from asubstantial chamfer I at the back of the casing to the face thereof, the

ated in continuous relative movement sliding friction engagement with afiat-face armature plate, having a. structure comprising, a circularcylindricalcasing of ferromagnetic'material providing a yoke andanannular winding spaceopen at the face of the casing between concentric Qcentral and outer cores, the depth and radial width of said windingspace being proportioned to provide a substantially uniform area ofmagnetic path in said casing, said proportioning .being determined byboth the outside diameter of said casing and the yoke thickness at thecenter line of the winding space, a non-magnetic gap of non-magneticmaterial 'interposed in the said magnetic path, and a separate workingfacing depth and radial width of said winding space being proportionedto provide a substantially uniform area of. magnetic path in saidcasing, said proportioning being determined by the radius of the saidopening in the central core and by both the outside diameter of saidcasing and the yoke thickness at the center line of the winding space, anon-magnetic gap of non-magnetic material interposed in the saidmagnetic path, and a separate working facing arranged over saidnonmagnetic material, said interposed gap being adapted to assure nicetyof electromagnet control and to prevent substantial residual magneticattraction between electroma net and armature.

4. A compact iron-clad central-core flat-face direct currentelectromagnet adapted to be operated in continuous relative movementsliding friction engagement with a flat-face armature plate comprising,a circular cylindrical casing of ferromagnetic-material providing a yokeand an annular winding space open at the face of the casing betweenconcentric central and outer cores, a winding arranged in said space, anonmagnetic gap of thin non-magnetic nietal arranged over the saidcentral core face, and a relatively thin disc-like circularfacinglmember of non-metal friction and magnetic materials affixed tosaid casing over the face thereof, said facing member having a smoothflat working surface of predetermined overall coefilcient of frictioncharacteristic and being adapted to'provide coplanar working magneticpole faces for said central and outer cores.

5. A compact iron-clad central-core flat-face magnetic materialproviding a yoke and an annular winding space open at the face of thecasing between concentric central and outer cores, a winding arranged insaid space and a relatively thin disc-like circular facing member ofnonmetal friction and magnetic materials aflixed to said casing over theface thereof, said facing member having a. smooth flat working surfaceof predetermined overall coefficient of friction characteristic andbeing adapted to provide coplanar working magnetic pole faces for saidcentral and outer cores, and non-magnetic material of predeterminedfixed thickness interposed in the ma netic path within said casing andfacing.

6. Acompact iron-clad central-core fiat-face direct currentelectromagnet adapted to be operated in continuous relative movementsliding friction engagement with a flat-face armature plate,

having a structure comprising, a circular cylindrical casing offerromagnetic material providing a yoke and an annular winding spaceopen at the face 'of the casing between concentric central and outercores, said central core having a circular opening therethroughconcentric the axis of the casing, a non-magnetic gap of thinnon-magnetic metal arranged over the said central core face, and arelatively thin circular disc-like facing member of non-metal frictionand magnetic materials aflixed to said casing over the facethereof,'said facing member providing a cover over said circular openingand coplanar working magnetic pole faces for said central and outercores, and having the characteristic infiat rubbing contact with thearmature of assuring a substantially constant coefiicient of frictionindependent of the rate of relative motion between the electromagnet andarmature and of the magnetic attraction normal force therebetween.

'7. A compact iron-clad central-core fiat-face direct currentelectromagnet adapted to be operated in continuous relative movementsliding friction engagement with a fiat-face armature plate comprising,a circular cylindrical casing of ferromagnetic materialproviding a yokeand an annular winding space open at the face of the casing betweenconcentric central and outer cores,

a preformed winding arranged in said space, a a

thin annulus of non-magnetic material arranged over said winding withinsaid winding space and adapted to present a substantially fiat surfacecontiguous said core faces, and a relatively thin circular disc-likefacing'member of non-metal friction and magnetic materials aflixed tosaid casing over the face thereof, said facing member providing a coverfor said annulus and coplanar working magnetic pole faces for saidcentral and outer cores and having the characteristic in fiat rubbingcontact relation with the armature of assuring a substantially constantcoeflicient of friction independent of the ratef of relative motionbetween the electromagne and armature and the magnetic attraction normalforce therebetween.

8. A compact iron-clad central-core fiat-face direct currentelectromagnet adapted to be operated in continuous relative movementsliding fricthin non-magnetic metal arranged over the 'said centralcore'face, and a relatively thin circular disc-like facing member havingalternate concentric annular segments of non-metal friction and magneticmaterials affixed to said casing over the,

face thereof, said facing member providing a cover for said circularopening and coplanar working magnetic pole faces for said central andouter cores and having the characteristic in flat rubbing contact withthe armature of assuring a substantially constant coefficient offriction independent of the rate of relative motion between theelectromagnet and armature and of the mag- .netic attraction normalforce therebetween.

9. A compact iron-clad central-core fiat-face direct currentelectromagnet adapted to be operated in continuous relative movementsliding friction engagement with a fiat-face armature plate, having astructure comprising, a circular cylindrical casing of ferromagneticmaterial providing a yoke and an annular winding space open at the faceof the casing between concentric central and outer cores, a relativelythin circular disc-like facing member 'of non-metal friction andmagnetic materials affixed to said casing over the face thereof, saidfacing member providing coplanar working magnetic pole faces for saidcentral and outer cores, and a thin sheet of nonmagnetic material ofuniform thicknessinterposed between at least one of sai core faces andthe underside of said ,facing, said interposed non-magnetic materialbeing adapted to establish a non-magnetic gap of predetermined fixedthickness in the magnetic path within said cas- .ing and facing.

10. In combination, acompact iron-clad central-core fiat-face directcurrent electromagnet in continuous relative movement sliding frictionengagement with a rotating disc-like flat-face armature, saidelectromagnet structure comprising a cylindrical casing of ferromagneticmaterial providing a yoke and a winding space open' at the face of thecasing between a central and an outer core, a relatively thin disc-likefacing amxecl to said casing over the face thereof providingcoplanarworking p'ole faces for said cores,

and a flat-face armature plate having working faces in continuous relatiovement sliding friction engagement, a non-magnetic gap of thinnon-magnetic ma netic path of s ctromagnet, said electromagnet includinga separate working facing arranged over said non-magnetic gap, saidnonmagnetic gap being disposed in non-contacting relation with both saidelectromagnet and arma ture working faces.

12. A fiat-face electromagnet adapted to be operated in continuousrelative movement sliding friction engagementwith a flat-face armatureplate and having a non-magnetic gap of nonmagnetic material interposedin the magnetic path within the electromagnet structure in substantialparallel non-contacting relation with the working pole-faces thereof. a

"'13; A flat-face electromagnet adapted to be operated in continuousrelative movement slidinterposed in the maging friction engagement witha flat-face armature plate and having a non-magnetic gap of non-magneticmaterial interposed in the magnetic path within the electromagnetstructur in non-contacting relation with the working faces thereof, saidinterposed gap being adapted to assure nicety of electromagnet controland to prevent substantial residual magnetic attractio'n betweenelectromagnet andarmature.

14. In combination, .9. compact iron-clad central-core flat-faceelectromagnet in continuous relative movement sliding frictionengagement with a flat-face armature plate, a non-magnetic gap of thinnon-magnetic material interposed in the magnetic path within the centralcore thereof in non-contacting relation with the central workingpole-face, said interposed gap being adapted to assure nicety ofelectromagnet control and to prevent substantial residual magneticattraction between electromagnetand armature.

15. In combination, a flat-face electromagnet and a flat-face armatureplate having working faces adapted to be operated in continuous relativemovement. sliding, friction engagement, at non-magnetic gap ofnon-magnetic metal interposed in the magnetic path of saidelectromagnet, said electromagnet including a separate working facingarranged over said non-magnetic metal gap, said non-magnetic metal gapbeing arranged in substantially parallel non-contacting relation withsaid electromagnet and armature working faces.

FRANK C. STOCKWELL; WILLIAM F. PENROSE.

